Depends on your needs. Most popular are Android and IOS. There are also Windows Mobile and Blackberry. Android is used more often but it’s perfect if modern technologies aren’t a trouble for you – it’s an open system ready to download and install anything you wish for. IOS (Apple) is a closed mobile OS and you’ll have to deal with some restrictions and payments, but you’ll get a stable system, cause it doesn’t give you a chance to interrupt with it’s functionality.

Windows Mobile is designed to look like Microsoft Windows. It’s quite a complex OS, it is more business oriented cause it’s good at content creation – it is compatible with powerful business software. Windows Mobile is often confusing for unexperienced users cause it’s least intuitive – the touch screen is a lot different from Android or IOS for example. But it gives you a fast working device with useful hardware features for experienced users.